1. Field of the Invention
This invention relates to an optical modulation system used for, for instance, an optical disc cutting apparatus for recording video or audio or data either alone or in combination on an optical video or audio disc or an optical data disc where data is stored.
2. Description of the Prior Art
A so-called optical system is usually used for recording a signal on a video disc or a digital audio disc. This system uses as a recording medium a glass or like disc with a thin and uniform coating of a photoresist (photochemical reacting agent). A laser beam modulated according to a recording signal in an optic modulator is given as a spot having a very small diameter to the photoresist to thereby record a signal through sensitization of the photoresist. Subsequently, the disc is developed.
In a usual optical disc cutting apparatus, a laser beam L.sub.o provided from a laser beam source is supplied to an optic modulator for modulation according to a recording signal, and the modulated light beam is supplied through a beam splitter or like optical system and then an objective lens or like lens system to be supplied as a spot having a very small diameter to the disc rotated from a motor or the like, whereby the signal is recorded on the disc, as disclosed in U.S. Pat. No. 4,225,873.
As the optic modulator, there is an electro-optic modulator disclosed in the Bell System Technical Journal, vol. 50, No. 8, Oct. 1971 "Optical Modulation at High Information Rates" by G. White, in which the refractive index of a crystal is varied according to an electric field applied thereto. Another modulator is disclosed in Proceedings of the IEEE, vol. 54, No. 10, Oct. 1966 "A Television Display Using Acoustic Deflector and Modulation of Coherent Light" by A. Korpel et al, which is a so-called acoust-optic modulator in which the refractive index of a medium is varied according to an acoustic sound wave. In these modulators, the light output has a characteristic of the square of a sine wave with respect to the voltage applied to the medium. The electro-optic modulator has a broader modulation bandwidth than the acoust-optic modulator, and optical disc cutting apparatus employing the electro-optic modulators have been well known in the art.
When information signal is recorded on a disc or the like in such the optical disc cutting apparatus, the exposure level, i.e., the output light intensity of the optic modulator, has to be accurately controlled to a prescribed level according to the information signal in conformity to the photosensitive characteristic of the photoresist or a metal film.
Meanwhile, the characteristics of the electro-optic modulator are subject to variations under the influence of heat generated in a crystal. In the optical disc cutting apparatus employing the electro-optic modulator, therefore, the intensity of the modulated laser beam is instable with respect to a prescribed value. This drawback may be obviated by introducing a feedback control system as disclosed in the U.S. Pat. No. 4,225,873, noted above.
However, if the characteristic of the electro-optic modulator is varied beyond a range over which the control by the feedback control system is possible, the output light of the electro-optic modulator can no longer be stablized.
The light intensity versus applied voltage characteristic of the electro-optic modulator is varied according to the temperature. When the characteristic is varied greatly, the polarity of the applied voltage has to be switched in case when the light output is changed in the positive direction and in case when it is changed in the negative direction. Heretofore, this switching is done with a manual switch. This means that the handling of the optic modulator is rather cumbersome.